1. Field of the Invention
The present invention relates to a powdered means for the cleaning of exhaust gases, e.g. gases exhausted from industrial smokestacks (smoke and chimney gases), employing the principle of dry adsorption. The powdered means generally includes a mixture of activated cokes with inert materials, which inert materials do not have any dust explosiveness.
2. Background Information
Activated cokes, such as activated carbon on a basis of hard coal or peat, or metallurgical cokes on a basis of lignite, are sometimes used with the admixture of inert materials for cleaning exhaust gas. Such mixtures have been used to remove acid pollutants (e.g. sulfuric acid, hydrochloric acid, hydrofluoric acid), organic substances such as chlorinated hydrocarbons (e.g. dioxins and furans), as well as heavy metals and heavy metal compounds from exhaust gas. In installations which function according to the airstream adsorption principle, such means are distributed as uniformly as possible in the current of smoke or chimney gas, react in flight when airborne with gaseous pollutants and other pollutants, and are precipitated on filters, along with other particulate constituents of the exhaust gas. The cleaned exhaust gas is then released through a chimney.
For the precipitation of acid pollutant gases, such as HCl SO.sub.2, SO.sub.3 and HF in the airstream process, reactive calcium compounds have been used in the past, such as finely-divided calcium hydroxides Ca(OH).sub.2. Calcium hydroxides for injection into the current of exhaust gases can be produced by various known processes, e.g. the process disclosed in German Patent DE 34 33 228.
Such known processes include the use of activated cokes, such as activated carbons and open hearth lignite cokes, for the removal of heavy metals, heavy metal compounds and organic compounds such as dioxins and furans, e.g. those which are found in the exhaust gases from municipal waste incineration plants.
One disadvantage of such processes is that when handling combustible adsorbents, even if the rules of the art are observed the use of activated cokes can entail the risk of dust explosions. In mixtures of inert materials with activated cokes. mixtures which are typically considered as non-explosive generally include at least mixtures with a concentration of carbon of less than or equal to 30% by weight. But, there are additional safety requirements which must typically be met for the use of these powdered means which include activated cokes and inert materials. Unless the mixtures are inherently safes precautionary measures to prevent explosions are essential, and such precautionary measures can significantly increase the cost of the airstream process. Under essentially no operating conditions may there be fluctuations in the concentration, as such fluctuations could form currents or even dust deposits which contain hazardous carbon concentrations.
In two-component mixtures, on the basis of reactive calcium compounds and powdered activated cokes, the granular structure of the individual components is so different that disruptive fluctuations in concentration are essentially unavoidable. With activated cokes, depending on the variety used, the maximum grain size which characterizes the granular structure can be between about 200 .mu.m and about 500 .mu.m, and with calcium hydroxide as the reactive calcium compounds the maximum grain size can be up to approximately 30 .mu.m.
If such mixtures of different granular structure, e.g. mixtures of calcium hydroxide and activated cokes are used in the airstream process in exhaust gas cleaning systems, local fluctuations in concentration can occur in the flow field at points where there are centrifugal forces, such as in elbows. The larger particles are then enriched in the coarse grain fraction, i.e. there are higher carbon concentrations there.
It is essentially impossible to prevent small amounts of the coarse grain fraction which have an enriched carbon concentration from precipitating out of the flow field, and in particular at points through which there is not a strong flow. Laboratory tests with mixtures of calcium hydroxide and activated coke in laboratory wind sifters have shown that even with initial mixtures which have carbon concentrations of less than 30%, carbon enrichment can occur. Such carbon enrichments are generally unacceptable from a safety point of view.
It has been determined that this problem can essentially not be solved by the conventional grinding of the activated coke, in which the amount of coarse grain is limited so that even the coarsest particles can become airborne under the conditions of the airstream process (where the gas velocities are as low as 6 m/s).
Attempts to match or adjust the grain size distribution of the powdered activated cokes to the calcium hydroxide, so that both components have essentially the same distribution of the speed of vertical descent or drift, have shown that, of course, with sufficiently fine grinding there are essentially no further fluctuations in the concentration. But, that would require that the activated carbons be ground down to approximately the grain size of the calcium hydroxide, i.e. to &lt;50 .mu.m. For technical reasons, grinding down activated carbons to match the grain size of calcium hydroxide would typically be difficult and time-consuming, as well as economically unacceptable.
Alternatively, if an attempt were made to agglomerate finely-divided calcium hydroxide to the point where the distribution of its-speed of vertical descent or drift matched the distribution of the speed of vertical descent or drift of the activated coke, the effectiveness of the mixture for the separation of acid pollutant gases would be significantly reduced, so that such a solution also does not seem logical.